1. Field of the Invention
The present invention relates in general to pneumatic springs of the cylinder-and-piston rod type and, in particular, to such pneumatic springs which include structure for selectively releasably retaining the piston rod at or adjacent the fully retracted position.
2. The Prior Art
Pneumatic springs which include a provision for retaining the piston rod at or near the fully retracted position within the cylinder cavity are known in the prior art. For example, U.S. Pat. No. 4,166,612 discloses a pneumatic spring in which the piston rod is provided with an extension at its inner end to sealingly engage a matching chamber in the interior of the cylinder as the piston rod reaches or nears its innermost position and thereby retain the piston rod at or near such innermost position. In this prior art device, the cylinder cavity communicates with the chamber at a point below the piston through valve means which opens as the piston rod extension enters the chamber and which closes when the piston rod starts to leave the chamber. Closure of the valve means functions to retain the extension within the chamber and, consequently, the piston rod at the innermost position within the cylinder. This known retaining structure is particularly suited for use in vehicles as auxiliary lifting means for the rear lid or trunk lid of a car, the lifting force being eliminated in the region of the inner end position of the piston rod by virtue of the action of the retaining structure. Although such prior art devices are useful for these purposes, it is desirable in certain situations that the retaining structure be operable only at certain times, as, for example, when the spring is being mounted or dismounted, and that the piston rod otherwise be able to move freely and fully inward and outward of the cylinder without engagement by the retaining structure. The aforementioned prior art devices, however, are not capable of such selective operation of the retaining structure.